/Engine/Source/Runtime/Engine/Classes/Components/AudioComponent.h
This function allows designers to call Play on an Audio Component instance while applying a volume curve over time. Parameters allow designers to indicate the duration of the fade, the curve shape, and the start time if seeking into the sound.
virtual void FadeIn ( float FadeInDuration, float FadeVolumeLevel, float StartTime, const EAudioFaderCurve FadeCurve )